The inductively coupled plasma is the most widely used plasma source in atomic spectrometry and pneumatic nebulization is the sample introduction system of choice for routine analysis. Despite their wide acceptance and applicability, pneumatic sample introduction systems drift, may block at high salt concentrations and require sample volumes larger than 1 mL. Perhaps the most important drawback of pneumatic nebulization is low sample introduction efficiency (typically 5% or less). Of the various alternative sample introduction systems that address the limitations of pneumatic nebulization, provide the capability for the analysis of .mu.L volumes of samples and offer increased sample introduction efficiency, direct sample insertion (DSI), (ref. 1, a list of references appears at the end of the specification) and electrothermal vaporization (ETV) sample introduction (ref. 2) will be considered.
In a typical DSI-device, a sample is deposited into or onto a probe, for example, a graphite cup or wire-loop, with subsequent introduction of the sample carrying probe into the plasma (see FIGS. 1a and 1b). Using DSI-devices, .mu.L volumes of liquids and mg quantities of solids can be introduced into the plasma with 100% sample introduction efficiency. Despite their advantages, DSIs are not without shortcomings. For example, refractory carbide formation is a key chemical limitation of graphite-cup DSIs. Further, since the plasma is used for vaporization, atomization and excitation, ICP and DSI-device operating conditions cannot be optimized independently.
A way to separate vaporization from atomization and ionization/excitation and to facilitate independent optimization is by using an ETV-device (see FIG. 1c). In a typical ETV sample introduction system, a sample is deposited or placed into or onto an electrically heated graphite or metal sample holder. The sample holder is heated using an expensive (approximately $20,000), microwave oven-size power supply and sample holders used with ICP-AES (ICP-atomic emission spectrometry) are furnaces, rods, cups, micro-boats and cuvettes in a graphite furnace, Ta filaments and Pt and W coils, W boats and W coils. The sample holder is placed into a volatilization chamber where the sample is heated to temperature between about 2700.degree. C. and 3000.degree. C. by passing electrical current through the sample holder and analyte vapor so generated is carried into the plasma by means of tubing and a carrier gas, typically Ar. The separation of vaporization (ETV-device) from atomization, excitation and ionization (ICP) facilitates independent optimization.
The advantages offered by ETV-ICP include increased sample introduction efficiency (resulting in improvements in detection limits when compared to pneumatic nebulization) and an inherent ability to handle small sample volumes (approximately 10 .mu.L). In addition to these advantages, a number of benefits accrue by coupling a ETV (or DSI) sample introduction to ICP-MS (ICP-mass spectrometry). In particular, spectral interferences, such as overlaps arising from polyatomic, oxide and hydroxide species resulting from continuous introduction of solvent are minimized because the solvent is vaporized prior to analyte vaporization. In addition, some non-spectroscopic interferences are minimized when analytes volatilize at different temperatures than the matrix.
Similar to DSIs, carbide formation is a key chemical limitation of graphite furnace ETV-devices. One way to eliminate carbide formation is by electrically heating metal rather than graphite. For instance, Ta filament and Pt and W coil, W boat and W coil ETV-devices have been coupled to ICP-AES and W ribbon, W filament, Re filament, Ta strip and Ta tube and W wire in graphite furnace ETV-devices have been coupled to ICP-MS.
In terms of non-chemical limitations, atomic vapor transfer problems, such as vapor-dilution and vapor-condensation onto the walls of the ETV chamber and the inner walls of the tube connecting the ETV-device to the ICP and transport effects, have been reported in the literature. Vapor transfer problems have been reduced by developing an ETV-device with a small-volume volatilization chamber, by minimizing the length of the tube connecting the ETV device to an ICP and by using an optimized chamber design. In addition, the relatively large mass of a typical graphite furnace ETV-device, for example, about 0.6 g for a graphite tube in graphite furnace atomic absorption spectrometry, causes rapid heating of the carrier gas which induces gas expansion and creates a transient increase in the carrier-gas flow-rate. This "pressure pulse" or "piston effect" causes a momentary decrease in plasma continuum emission and complicates background correction. The use of a lower temperature, e.g. below about 1400.degree. C. versus a typical greater than about 2700.degree. C., smaller surface area ETV-device, an increase in the length of tubing connecting the ETV-device to the ICP, an increase in the observation height and in the carrier gas flow rate, a reduction in the volume of the volatilization chamber and the use of an optimally designed chamber and a double wall chamber, have been reported to reduce the adverse effects of the pressure pulse.
Partially due to the relatively large mass and the low electrical resistance (ca. 15 m.OMEGA.) of graphite furnaces, the electrical power requirement is about 2 kW, thus necessitating the use of a bulky and relatively expensive power supply that has special power requirements. Despite the improvements in detection limits and the benefits of coupling ETV to ICP-MS, these shortcomings limit wide acceptance and applicability of ETV-ICP.